1. Statement of the Technical Field
The inventive arrangements relate generally to methods and apparatus for providing increased design flexibility for RF circuits, and more particularly to controlling modes within a waveguide.
2. Description of the Related Art
A waveguide typically includes a material medium that confines and guides a propagating electromagnetic wave. In the microwave regime, a waveguide normally consists of a hollow metallic conductor, usually rectangular, elliptical, or circular in cross-section. This type of waveguide may, under certain conditions, contain a solid or gaseous dielectric material.
In a waveguide or cavity, a “mode” is one of the various possible patterns of propagating or standing electromagnetic fields. Each mode is characterized by frequency, polarization, electric field strength, and magnetic field strength. The electromagnetic field pattern of a mode depends on the frequency, refractive indices or dielectric constants, and waveguide or cavity geometry. With low enough frequencies for a given structure, no mode will be supported. At higher frequencies, higher modes are supported and will tend to limit the operational bandwidth of a waveguide. Each waveguide configuration can form different modes of operation. The easiest mode to produce is called the Dominant Mode. Other modes with different field configurations may occur accidentally or may be caused deliberately. Hence, it may be desirable to suppress certain higher modes by providing a particular waveguide structure that slightly attenuates in a desired mode while significantly attenuating an undesired mode or modes.
An “evanescent field” in a waveguide is a time-varying field having an amplitude that decreases monotonically as a function of transverse radial distance from the waveguide, but without an accompanying phase shift. The evanescent field is coupled, i.e., bound, to an electromagnetic wave or mode propagating inside the waveguide. In other words, an evanescent mode can be a signal below a cut-off frequency that propagates through the waveguide to a given extent and becomes weaker as it traverses through the waveguide.
Variable waveguide attenuators are commonly used to attenuate microwave signals propagating within a waveguide, which is a type of transmission line structure commonly used for microwave signals. Waveguides typically consist of a hollow tube made of an electrically conductive material, for example copper, brass, steel, etc. Further, waveguides can be provided in a variety of shapes, but most as previously mentioned often are cylindrical or have a rectangular cross section. In operation, waveguides propagate modes above a certain cutoff frequency.
Waveguide attenuators are available in a variety of arrangements. In one arrangement, the waveguide attenuator consists of three sections of waveguide in tandem: a middle section and two end sections. In each section a resistive film is placed across an inner diameter of the waveguide (in the case of a waveguide having a circular cross section) or across a width of the waveguide (in the case of a waveguide having a rectangular cross section). In either case, the resistive film collinearly extends the length of each waveguide section. The middle section of the waveguide is free to rotate radially with respect to the waveguide end sections. When the resistive film in the three sections are aligned, the E-field of an applied microwave signal is normal to all films. When this occurs, no current flows in the films and no attenuation occurs. When the center section is rotated at an angle θ with respect to the end section at the input of the waveguide, the E field can be considered to split into two orthogonal components, E sin θ and E cos θ. E sin θ is in the plane of the film and E cos θ is orthogonal to the film. Accordingly, the E sin θ component is absorbed by the film and the E cos θ component is passed unattenuated to the end section at the output of the waveguide. The resistive film in the end section at the output then absorbs the E cos θ sin θ component of the E field and an E cos2 θ component emerges from the waveguide at the same orientation as the original wave. The accuracy of such an attenuator is dependant on the stability of the resistive films. If the resistive films should degrade over time, performance of the waveguide attenuator will be affected. Further, energy reflections and higher-order mode propagation commonly occur in such a waveguide attenuator design.
In another arrangement, a wedge shaped waveguide attenuator having resistive surfaces exists. Because the waveguide attenuator is wedge shaped, the E field again can be considered to split into two orthogonal components at each surface of the wedge, E sin θ and E cos θ. As with the previous example, the E sin θ component of a microwave signal is absorbed by the film. However, the tapered portion of the waveguide attenuator causes energy reflections to occur. Hence, the wedge shaped waveguide attenuator must be long enough to obtain sufficiently low reflection characteristics. Accordingly, this type of waveguide attenuator is limited to use in relatively long waveguides. Thus, a need exists for a waveguide and a waveguide attenuator that provides additional design flexibility and overcomes the limitations described above with respect to existing waveguides and waveguide attenuators.
A waveguide will have field components in the x, y, and z directions. A waveguide will typically have waveguide dimensions of width, height and length represented by a, b, and l respectively. There are no z-directed currents in the short walls of the waveguide (either for propagating mode or evanescent mode), so the short wall does not need to be continuous in the z-direction. Thus, an array of vertical (y-directed) wires would alternatively work as well. The cutoff frequency or cutoff wavelength (for transverse electric (TE) modes) can be represented as:             (              f        c            )              m      ⁢                           ⁢      n        =            1              2        ⁢                                   ⁢        π        ⁢                              μ            ⁢                                                   ⁢            ɛ                                ⁢                                        (                                          m                ⁢                                                                   ⁢                π                            a                        )                    2                +                              (                                          n                ⁢                                                                   ⁢                π                            b                        )                    2                    and             (              λ        c            )              m      ⁢                           ⁢      n        =      2                                        (                          m              a                        )                    2                +                              (                                          n                ⁢                                                                               b                        )                    2                    where a, b are waveguide dimensions as shown in FIG. 5, c is the speed of light, ε and μ describes the dielectric inside the waveguide and m, n are mode numbers. The lowest frequency mode TE10 (m=1, n=0) is also known as the dominant mode and provides the most efficient mode for propagation. The dominant mode for rectangular waveguides is designated as the TE mode because the E fields are perpendicular to the “a” walls. The first subscript is 1 since there is only one half-wave pattern across the “a” dimension. There are no E-field patterns across the “b” dimension, so the second subscript is 0. The complete mode description of the dominant mode in rectangular waveguides is TE1,0. Waveguides are normally designed so that only the dominant mode will be used. To operate in the dominant mode, a waveguide must have an “a” (wide) dimension of at least one half-wavelength of the frequency to be propagated. In rectangular waveguides, the first subscript indicates the number of half-wave patterns in the “a” dimension, and the second subscript indicates the number of half-wave patterns in the “b” dimension. The “a” dimension of the waveguide must be kept near the minimum allowable value to ensure that only the dominant mode will exist. In practice, this dimension is usually 0.7 wavelength. The high-frequency limit of a rectangular waveguide is a frequency at which its “a” dimension becomes large enough to allow operation in a mode higher than that for which the waveguide has been designed. Thus, a need exists to dynamically adjust the dimension of a waveguide in certain scenarios.
The field arrangements of the various modes of operation are divided into two categories: Transverse electric (TE) and Transverse Magnetic (TM). In the transverse electric (TE) mode, the entire electric field is in the transverse plane, which is perpendicular to the length of the waveguide (direction of energy travel). Part of the magnetic field is parallel to the length axis. In the transverse magnetic (TM) mode, the entire magnetic field is in the transverse plane and has no portion parallel to the length axis. Since there are several TE and TM modes, subscripts are used to complete the description of the field pattern.
A similar system is used to identify the modes of circular waveguides. The general classification of TE and TM is true for both circular and rectangular waveguides. In circular waveguides the subscripts have a different meaning. The first subscript indicates the number of full-wave patterns around the circumference of the waveguide. The second subscript indicates the number of half-wave patterns across the diameter. In the circular waveguide, the E field is perpendicular to the length of the waveguide with no E lines parallel to the direction of propagation. Thus, it must be classified as operating in the TE mode. If you follow the E line pattern in a counterclockwise direction starting at the top, the E lines go from zero, through maximum positive (tail of arrows), back to zero, through maximum negative (head of arrows), and then back to zero again. This is one full wave, so the first subscript is 1. Along the diameter, the E lines go from zero through maximum and back to zero, making a half-wave variation. The second subscript, therefore, is also 1. TE1,1 is the complete mode description of the dominant mode in circular waveguides. Several modes are possible in both circular and rectangular waveguides.